Wireless data networks such as WLANs are becoming increasingly popular due to their many advantages over wired networks. They provide all the functionality of wired networks without the physical constraints. Although wireless networks can be more costly to install initially, the installation is often quicker and less disruptive to the work environment then for wired networks. Once installed they provide greater physical mobility within the network area for users, which can in some environments in particular provide for much greater productivity. In addition wireless networks can be expanded and altered much more readily than wired networks and thus are more readily adapted to changing requirements than is the case for wired networks.
Wireless networks use radio waves, or in some cases infra red, to communicate information from one point to another without the need for any physical connection. For example a typical WLAN configuration comprises a transmitter/receiver (transceiver) device incorporating an antenna, commonly called an access point, connected to a wired network at a fixed location. The transceiver receives, buffers, and transmits data between the WLAN and the wired network infrastructure. End users access the WLAN through WLAN adapters which are implemented as PC cards in notebook computers, or use ISA (industry standard architecture) or PCI (peripheral component interconnect) adapters in desktop computers, or fully integrated devices within hand held devices such as personal digital assistants (PDAs). The WLAN adapters provide an interface between the network operating system and the radio waves, via an antenna. The nature of the wireless connection is transparent to the network operating system.
As illustrated schematically in FIG. 1, which shows a prior art WLAN, in many WLANs such as WLAN 10 there are a number of access points 12 to a wired network infrastructure 14 in order to provide the appropriate physical coverage, e.g. a whole building 16, or campus. The access points 12 not only provide communication with the wired network infrastructure 14 but also mediate wireless network traffic in the immediate neighbourhood. The area covered by each access point 12 is often referred to as a microcell 18, and these are illustrated by broken lined circles. At any time a device, or node, equipped with a WLAN adapter and accessing the WLAN is associated with a particular access point 12 and its microcell 18. If that device is moved within the coverage of the WLAN then it may move into a different microcell and become associated with a different access point.
If the antennae used by the access points 12 are not directional the area covered by a microcell 18 is approximately circular, (although this will be affected by the environment in which it is located which can produce reflections etc. which alter the basic coverage). Thus to provide fill coverage of an operational area such as a building 16, or campus, by a WLAN the microcells 18 are configured to overlap with each other and with the edge of the area, i.e. building 16, which the WLAN 10 must cover. This provides a security problem, as the coverage of the WLAN 10 extends outside the building 16 potentially providing areas 20, shown shaded in FIG. 1, which may be outside a secure area to which access can be limited and thus provides areas where eavesdroppers may locate a device and seek to gain access to the WLAN 10 and thus to the wired network infrastructure 14 as a whole. Although other security measures may be in place, such as access keys, passwords, encryption etc. these are not infallible, hence it would clearly be s preferable to minimise the areas 20 outside the building or secure area to which the WLAN extends. For simplicity such areas will be referred to in this specification as prohibited areas.